Wireless communication networks, such as 4th generation (4G) mobile networks, 5th generation (5G) mobile networks, etc., allow User Equipment (UE) mobile devices to communicate with each other and with other devices via a radio access network (RAN) infrastructure. Networks conforming to the Long Term Evolution (LTE) standard are considered to be an implementation of 4G networks. The RAN infrastructure can be embodied in base stations and includes for example antennas, radiofrequency and baseband processing devices, etc. In LTE networks, much of this infrastructure is the found in the evolved NodeB (eNB).
A trend in RAN design is to spatially distribute the different RAN functionalities. For example, distributed base stations in which the remote radio head (RRH) is separated from the baseband unit (BBU) by a fiber-optic link have been implemented for 3rd generation (3G) mobile networks. The fronthaul network refers to the communications network used to connect the BBU and the RRH. Cloud-based or centralized RAN (C-RAN) solutions have been proposed in order to provide for large-scale centralization of BBU resources in a resource pool, connected to multiple RRH units in the field.
However, the spatial distribution of RAN elements, such as BBU and RRH elements, leads to various implementation issues. For example, the fronthaul network can introduce unknown and potentially time-varying latencies, which can reduce phase error between different RRHs. Excess phase error can impact operational requirements such as air interface orthogonality requirements, coordinated multipoint (CoMP) requirements, enhanced inter-cell interference coordination (eICIC) requirements, Carrier Aggregation technology requirements, multi-input multi-output MIMO communication system requirements, etc. For example, CoMP with joint transmissions from multiple RRHs requires phase synchronization within an accuracy of ±1.5 μs.
Various techniques for maintaining RRH phase alignment include: using GPS signals to synchronize different RRH internal clocks, or using a protocol such as IEEE 1588 (PTP), possibly with SyncE, for synchronization. An RRH can also receive some timing information from the common public radio interface (CPRI), if used. However, such approaches can be complex and/or inadequate in a given situation. For example, GPS signals may not be available for use by an RRH deployed in an indoor environment.
Therefore, there is a need for a method and apparatus that obviates or mitigates one or more limitations of the prior art.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.